The present invention relates to new reagents and a new permeabilization process for erythrocytes as well as its use in the identification of foetal erythrocytes by a new immuno-marking method.
Methods for the cytometric analysis of foetal erythrocytes after marking with fluorescent antibodies inside the erythrocytes already exist.
An antibody and its method is sold by the company BIOATLANTIQUE in Nantes (France) under the names xe2x80x9cAnticorps monoclonal anti-hemoglobine foetalexe2x80x9d and xe2x80x9cProtocole pour la cytomxc3xa9trie de fluxxe2x80x9d. This method has the drawback of being lengthy and complicated with twelve washes and 16 hours of incubation.
Another antibody and method is sold by CALTAG LABORATOIRES in Burlingame, Calif. (USA) under the names xe2x80x9cMonoclonal antibodies to human foetal hemoglobinxe2x80x9d and xe2x80x9cAnti-HbF-flow cytometric protocolxe2x80x9d. This method is shorter with six washes, but uses glutaric aldehyde as a fixing agent, which is not stable at ambient temperature.
It would thus be desirable to have reagents for erythrocytic permeabilization which were stable and capable of being stored for more than a year, and a process for erythrocytic permeabilization which did not contain more than four washing stages and a short incubation, for example thirty minutes.
Now, after lengthy research, the Applicant has discovered that a permeabilization of erythrocytes, without substantial loss of haemoglobin from the cell, can be obtained by treating erythrocytes using a fixing agent and a permeabilizing agent.
For this reason, a subject of the present invention is a process for the permeabilization of erythrocytes, characterized in that the erythrocytes are subjected successively to the action of
(a) a fixing agent containing an aliphatic aldehyde and an oligosaccharide,
(b) a permeabilizing agent containing a detergent and an oligosaccharide.
This new process allows an antibody to enter the erythrocyte and an intracellular antigen to be analyzed. The antigens present in the erythrocyte and particularly those which are carried by the haemoglobin are used for distinguishing and identifying foetal erythrocytes.
The fixing agent contains an aliphatic aldehyde, an oligosaccharide and preferably a sulphated polysaccharide. It is notably in an approximately isotonic or hypertonic and preferably slightly acidic medium.
The aliphatic aldehyde, preferably one containing C1-C5, can be for example paraformaldehyde and particularly formaldehyde.
The aliphatic aldehyde can be present in a concentration of 0.3M to 12.3M, in particular 1.5M to 10M and very particularly 3M to 8.3M. Under preferential conditions of use of the above fixing agent, 6.7M of formaldehyde are used.
The fixing agent also contains an oligosaccharide, preferably a disaccharide. As disaccharides, there can be mentioned for example saccharose, cellobiose, maltose, lactose, gentiobiose, melibiose and in particular trehalose.
The oligosaccharide can be present in a concentration of 0.025M to 1.32M, in particular 0.132M to 1.19M and most particularly 0.26M to 1.06M. Under preferential conditions, the fixing agent contains approximately 0.8M of trehalose.
A sulphated polysaccharide is advantageously used to avoid aggregations including nucleic acids which allows the background noise to be reduced when the above process is used in a process for detecting erythrocytes.
It is found in the fixing agent in a concentration between 0.1 mg/ml and 10 mg/ml. The fixing agent preferably contains approximately 1 mg/ml of dextran sulphate with a molecular weight in the region of 500,000.
The pH of the fixing agent is for example a pH between 3 and 8, in particular between 4 and 7. It is preferably slightly acidic, most particularly between 5 and 6. Under particular conditions, the fixing agent has a pH of approximately 5.5, preferably buffered by a 10 mM solution of sodium dihydrogen phosphate.
The isotonicity of the fixing agent is preferably obtained by the presence of a concentration of 0.15M NaCl.
The permeabilizing agent contains a zwitter-ionic detergent and in particular an anionic detergent. This anionic detergent is for example sodium dioxycholate or N-lauryl sarcoside, and in particular sodium dodecyl sulphate.
The concentration of detergent can be between 0.001% and 10%, and particularly between 0.01 and 5%. Under preferential conditions, the permeabilizing agent contains approximately 0.03% of sodium dodecyl sulphate.
In addition, the permeabilizing agent contains an oligosaccharide, particularly trehalose as described in the case of the fixing agent.
The concentration of trehalose in the permeabilizing buffer is situated for example between 0.0026M and 0.26M, preferably between 0.026M and 0.13M, and most particularly approximately 0.053M.
The pH of the permeabilizing agent is preferably slightly acidic with a pH between 3 and 8, in particular between 5 and 6. The permeabilizing agent has a pH of approximately 5.5 under particular conditions.
The pH of the permeabilizing agent is preferably buffered, advantageously by using 10 mM of sodium dihydrogen phosphate and 10 mM of sodium citrate.
The permeabilizing buffer is preferably approximately isotonic, for example by using a concentration of 0.15M of sodium chloride.
The process according to the invention can be used in particular in the identification of foetal erythrocytes by a new method of immuno-marking using antibodies. For this reason, after fixing and permeabilizing, the erythrocytes are advantageously washed mainly in order to eliminate the detergent which could have a harmful influence on the antibodies used for the revelation of antigens inside the erythrocyte.
The washing agent is preferably a solution having a pH between 3 and 8, and advantageously slightly acidic, between pH 5 and 6. This solution can contain a neutral detergent and can contain a strong acid and base salt such as NaCl and KClI. Preferably, the washing agent does not contain detergent and is hypotonic. Under particularly preferred conditions, the washing agent is distilled or demineralized water.
A subject of the present Application is also a new process for revealing foetal erythrocytes amongst a population of erythrocytes in adult blood.
The best-known method is that of Kleihauer, based on a better resistance of foetal erythrocytes to a hypotonic lysis. Under certain hypotonic conditions, a leaking-out of the haemoglobin of adult erythrocytes is observed, but not foetal erythrocytes, which allows foetal erythrocytes to be distinguished by colour under microscope inspection.
The drawback of this method is that in order to detect a small number of foetal erythrocytes, a large number of erythrocytes must be passed under the microscope, which represents a significant amount of handling. Also, certain pathologies can lead to a modification of the erythrocytes, resulting in a number of false positives. In addition, according to certain reports, this test would not be easy to reproduce.
Other methods are based on the permeabilization of erythrocytes and immuno-marking of foetal haemoglobin (g-haemoglobin) inside foetal erythrocytes with a fluorescent antibody. The latter method allows counting by cytometry but it has the drawback that certain erythrocytes of adult origin can contain foetal haemoglobin (F cells). Equally, in certain diseases such as thalassaemia, the number of adult erythrocytes containing foetal haemoglobin is increased.
Thus, it would be desirable to have a process for immuno-marking foetal erythrocytes which is highly sensitive with reduced possibilities for error.
Thus, a subject of the invention is also the above permeabilization process, characterized in that in addition, (d) the erythrocytes are reacted with two antibodies against two foetal antigens which are expressed on the erythrocyte independently. One, the foetal haemoglobin, is found inside the cell, the other, the i antigen, is found outside the cell.
In the course of this immuno-marking step, the erythrocytes, preferably suspended in water, are advantageously mixed with an equal volume of phosphate buffer containing 5 mg/ml of bovine serum albumin and the two antibodies. During this step, the pH becomes neutral, allowing the antibody to function well and the erythrocyte environment remains slightly hypotonic, allowing a good penetration of antibodies.
The simultaneous immuno-marking of foetal haemoglobin and i with the antibodies marked with two different fluorescent markers allows foetal erythrocytes to be counted as distinct from the xe2x80x9cF cellsxe2x80x9d.
The marking of the antibodies with the fluorescent markers such as the fluorescein N-isothiocyanate or phycoerythrin for example can be carried out conventionally as described for example by G. T. HERMANSON in Bioconjugate Techniques, chapter 8.1.1, Fluorescein Derivatives, pages 302-305, Academic Press 1996 or chapter 8.1.7., Phycobiliprotein Derivatives, pages 362-364. The desired antibody can in particular be conjugated with phycoerythrin activated with succinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate after reduction with 1 mM of DL-dithiothreitol in PBS.
Under preferential implementation conditions for the process described above, monoclonal antibodies of type IgG1 NaM16-2F4 deposited at the Collection Nationale de Microorganismes (CNCM) in Paris on Jun. 23, 1998 under No. 1-2044 are used for marking foetal haemoglobin, and the monoclonal antibody of type IgM NaM61-1A2 deposited at the Collection Nationale de Microorganismes (CNCM) in Paris on Jun. 23, 1998 under No. I-2043 is used for labelling the i antigen.
Under other preferential implementation conditions for the invention, the sensitivity of the test can be increased by adding a marker for the nucleic acids in a third colour.
In three-colour cytometry, the events corresponding to a false xe2x80x9cbackground noisexe2x80x9d, often leucocytes or dead cells which are rich in DNA, can thus be eliminated.
Under particularly preferred conditions, the marker used for the nucleic acids is a nucleated cell colorant, for example by fixing on the DNA, particularly LDS 751 (MOLECULAR PROBES Inc. Or, USA).
A further subject of the present Application is an erythrocyte permeabilization kit, characterized in that it includes
(a) an approximately isotonic fixing agent containing an aliphatic aldehyde and an oligosaccharide,
b) a permeabilizing agent containing a detergent and an oligosaccharide.
A further subject of the present Application is a foetal erythrocyte immunomarking kit, characterized in that it contains the above erythrocyte permeabilizing kit as well as two antibodies against two foetal antigens which are expressed independently on the erythrocyte.
Finally, a subject of the present application is a process for identifying foetal erythrocytes by immuno-marking, characterized in that the erythrocytes are successively subjected to the action of:
(a) a fixing agent containing an aliphatic aldehyde and an oligosaccharide in an approximately isotonic or hypertonic medium,
(b) a permeabilizing agent containing a detergent or an oligosaccharide operated according to claim 8
(c) a washing agent mainly in order to eliminate the detergent
(d) two antibodies against two foetal antigens which are expressed independently on the erythrocyte
then the of foetal erythrocytes are detected.
This detection can be carried out by all the well-known methods in the prior art for detecting fluorescent compounds, combined with the use of measurement thresholds or measurement scales.
The preferential conditions for implementing the processes described above apply equally to other objects of the invention described above, in particular the kits.
The following examples illustrate the present Application.
6.67M formaldehyde
0.8M D (+) trehalose
0.15M sodium chloride
0.01 M sodium dihydrogen phosphate
1 mg/ml dextran sulphate (PM 500.000)
pH 5.5
0.001M sodium dodecyl sulphate
0.15M NaCl
0.01M sodium dihydrogen phosphate
0.01M sodium citrate
0.053M D(+) trehalose
pH 5.5
Four samples of blood were selected to be subjected to a permeabilization procedure.
The samples were used with ethylene diamine tetraacetic acid (EDTA) as an anticoagulant.
1. Blood of a male subject.
2. One millilitre of the same blood described in 1 to which 0.01 ml of blood from the umbilical cord taken during the birth of a baby is added.
3. Blood of a pregnant woman whose blood has been examined by the Kleihauer method and for which the result obtained was negative.
4. Blood of a pregnant woman whose blood has been examined by the Kleihauer method and for which the result obtained was positive, giving a value of 0.1% of foetal erythrocytes.
Each sample of 0.01 ml of blood was mixed with 0.1 ml of fixing reagent (Example 1) and the mixture was incubated for 30 minutes at ambient temperature (20-25xc2x0 C.). After incubation, 3 ml of phosphate buffer (PBS=Phosphate Buffered Saline) was added as a washing agent and the whole was centrifuged at 300 g.
After aspiration, the cells were taken up in 3 ml of permeabilizing reagent (Example 1) and centrifuged at 300 g.
After aspiration, the cells were taken up in 1 ml of demineralized water.
20 xcexcl of each of the preparations of Example 2 of erythrocytes suspended in water were mixed with 20 xcexcl of the following preparation:
PBS containing:
50 xcexcg/ml of monoclonal antibodies NaM16-2F4 directed against foetal haemoglobin and conjugated with fluorescein isothiocyanate as described by G. T. HERMANSON in Bioconjugate Techniques, chapter 8.1.1, Fluorescein Derivatives, pages 302-305, Academic Press 1996;
3.12 xcexcg/ml of monoclonal antibodies NaM61-1A2 directed against antigen i and conjugated with phycoerythrin as described by G. T. HERMANSON in Bioconjugate Techniques, chapter 8.1.7., Phycobiliprotein Derivatives, pages 362-364;
3.12 xcexcg/ml of LDS 751 (Molecular Probes, OR, USA).
5 mg/ml of bovine serum albumin (BSA).
After incubation for 15 minutes, 3 ml of phosphate buffer containing 0.17M of formaldehyde was added to each mixture as a washing agent and centrifuged at 300 g for 5 minutes. After aspiration, the cells were taken up in 0.5 ml of phosphate buffer with 0.17M formaldehyde.
The hybridoma corresponding to the monoclonal antibody NaM16-2F4 was deposited at the Collection Nationale de Microorganismes (CNCM) in Paris on Jun. 23, 1998 under No. I-2044. The monoclonal antibody NaM16-2F4 is of IgG1 type. The hybridoma corresponding to the monoclonal antibody NaM61-1A2 was deposited at the Collection Nationale de Microorganismes (CNCM) in Paris on Jun. 23, 1998 under No. I-2043. The monoclonal antibody NaM61 is of IgM type.
Example 4
A Coulter XL cytometer (Miami, Fla., USA) was used for analysing the results. The fluorescent adjustments FL1 and FL2 and the compensations were obtained according to the manufacturer""s instructions using a reference sample xe2x80x9cCytotrolxe2x80x9d (Coulter No. Cat 6604248), mixed with 20 xcexcl of sample No. 1 of Example 2, and marked with a mixture of CD4-FITC(fluorescein N-isothiocyanate) and CD8-PE (phycoerythrin) (Immunotech, Cat No. IMO747).
The adjustment and compensation of FL4 was obtained with sample 2 of Example 2 after immuno-marking according to Example 3.